1. Field of the Invention
The present invention relates to a color picture information transmission system.
2. Description of the Prior Art
For transmitting a picture information such as video signals, for example, a variable density sampling system has been proposed wherein a sampling frequency is varied adaptively according to a frequency of the video signal for transmission. FIG. 1 is a pattern drawing of a principle of the variable density sampling system.
In FIG. 1, first an original video signal waveform is sampled according to a sampling frequency fsp, divided at every predetermined sampling point number (4 sampling points, for example) as indicated by a term T in the drawing, and then whether the original video signal waveform is sparse or dense is discriminated at every divided blocks. Then, in the block decided to be dense, all data obtained through sampling the original video signal is transmitted as transmission data, but in the block decided to be sparse, only a part of all data is transmitted as transmission data, and the remaining data is held as thinned-out data and hence is not transmitted. In the drawing, that with a mark C is a data to be transmitted (transmission data), and that with a mark x is a data not transmitted (thinned-out data).
The number of data to be transmitted per unit time will be decreased and a transmission band of the transmission video signal will be compressed by transmitting the data indicated by the mark O at constant intervals.
The thinned-out data having not been transmitted is restored approximately by means of the transmitted data, thus obtaining an interpolation data (with a mark in the drawing). Then, the interpolation data corresponds to a portion sparse in the original video signal waveform and is restored as a data extremely approximate to the thinned-out data, therefore substantial information content does not change as compared with the case where all data are transmitted, and thus a transmission band of the video signal may be sharply compressed.
In this case, all data are transmitted in each group, however, whether or not a part of data is transmitted will be decided upon investigating of the density of the original video signal waveform, and the decision information is transmitted as a transmission mode information signal at the same time.
FIG. 2 is a block diagram for realizing the aforementioned transmission system.
A video signal (or NTSC signal, for example) inputted from an input terminal is digitized by an analog-to-digital (A/D) converter 2. The sampling frequency fsp in this case is specified to be double or more of a maximum frequency component of the input video signal. An output signal of the A/D converter 2 is fed to a decoder 3 and separated into a luminance signal Y and two kinds of chrominance signals I and Q. The luminance signal Y is fed to a mode discriminating circuit 16 and also introduced to a matrix circuit 4, and the signals I, Q are fed to the matrix circuit 4 likewise. In the matrix circuit 4, three-primary color signals R, G, B are obtained through arithmetic operation according to each of the inputted signals Y, I, Q.
A relation between the signals R, G, B and the signals Y, I, Q stands as follows: EQU Y=0.30R+0.5G+0.11B EQU I=0.74(R-Y)-0.27(B-Y) EQU Q=0.48(R-Y)+0.41(R-Y)
The three-primary color signals R, G, B obtained from the matrix circuit 4 are loaded in R-, G-, B-memories 10, 11, 12 of a first memory group respectively as signals having a 1/2 fsp band by the matrix circuit 4 and also fed to a prefilter 6. For example, in the case of unidimensional variable density sampling in one block construction of the four sampling points like FIG. 1, the band is limited further to a quarter of 1/2 fsp through the prefilter 6. An output of the prefilter 6 has information of the three sampling points other than one sampling point inputted first of information of the four sampling points removed by a thinning-out circuit 5. The three-primary color signals having the band compressed to a quarter as above are loaded in R-, G-, B-memories 7, 8, 9 of a second memory group.
Outputs of the first and second memory groups are led to three input terminals of switch circuits 13 and 14, and further outputs of the switch circuits 13 and 14 are led to input terminals E, C of a switch circuit 15. An operation of the mode discriminating circuit 16 will be described according to FIG. 3 and FIGS. 4(a) to 4(d).
FIG. 4(a) represents a luminance signal row sampled by the sampling frequency fsp. A luminance signal at time t.sub.n is indicated by y.sub.n, a luminance signal at time t.sub.n+1 is indicated by y.sub.n+1, then y.sub.n+2, y.sub.n+3 and so on. Now, let it be assumed that y.sub.n to y.sub.n+3 constitute one unit block.
The luminance signal y.sub.n led to the mode discriminating circuit 16 is latched in a latch circuit 20 at the leading edge (t.sub.n) of a clock 1 shown in FIG. 4(b). An output of the latch circuit 20 is led to a latch circuit 21 and an interpolation circuit 22. The output of the latch circuit 20 is latched at a leading edge (t.sub.n+4) of the next pulse of the clock 1 by the latch circuit 21. As in the case of the thinning-out circuit 5, the luminance signals y.sub.n, y.sub.n+4 of the sampling point inputted first of the luminance signal unit block are latched on the latch circuits 20 and 21. Then, each output is led to the interpolation circuit 22.
The interpolation circuit 22 computes and outputs interpolation luminance signals y.sub.n to y.sub.n+3 at times t.sub.n to t.sub.n+3 from two inputs y.sub.n, y.sub.n+4 through a primary interpolation as shown, for example, in a broken line of FIG. 4(d).
On the other hand, the luminance signal y.sub.n is also inputted to a shift register 23 and shifted by leading edges (t.sub.n, t.sub.n+1, t.sub.n+2, . . . ) of the clock 2 shown in FIG. 4(c). As described, the luminance signal row led to the mode discriminating circuit 16 is output by the interpolation circuit 22 and the shift register 23 at the time t.sub.n+4 as shown in FIG. 3.
Each output works as an input to a subtractor 24, and the differential is led to an adder 26 by way of an absolute value circuit 25. An output of the adder 26 works as one input to a comparator 27 and is compared with a threshold value Yth preset on the other input.
As described above, in the mode discriminating circuit 1, a level of the luminance signal or a difference in power between information of the original sampling points constituting a block unit and information of the sampling points by interpolation is detected, and if the sum exceeds a threshold value, a transmission mode of the block is decided to be a mode (hereinafter called "E mode") for transmitting information of all the sampling points, but if it comes below the threshold value, then the transmission mode is decided to be a mode (hereinafter called "C mode") for transmitting information of the thinned-out sampling points, and thus a mode information data is generated by a mode information generator 28 according to the decision. The transmission mode information data thus generated is loaded once in a mode memory 17 of FIG. 2. Then, at the time of transmission, the switch circuits 14 and 13 are first connected to a terminal R side in the drawing. The circuits operate to connect the switch circuit 15 to the first memory group side when E mode but to the second memory group side when C mode according to the transmission mode information loaded in the mode memory 17, and a signal R read by the memory is transmitted through a transmission part 18 together with the mode information data. Then, whenever the signal R has been transmitted, the switch circuits 13, 14 are transferred one after another to G side and B side to transmit the signals G and B through the transmission part 18 likewise.
Meanwhile, according to the prior art transmission system described as above, a mode to transmit is discriminated by the mode discriminating circuit 16 only on a level of the luminance signal or a difference in power, therefore the video signal transmitted according to such transmission system is capable of causing an error in a hue from a band compression operation particularly in the block small in a change of the luminance signal but large in a change of the hue, and since such error may cause unevenness of color or the like, it is difficult to transmit the video signal with a picture quality thoroughly ensured.
Then, in case the video signal transmitted according to such transmission system as described is displayed on a display, since a human visual system characteristic is keen to a change in luminance in a low luminance portion as compared with a high luminance portion, a visual system distortion is intensified in the low luminance block as compared with the high luminance block, and thus an image after transmission seems to be degraded considerably.
Further, the video signal transmitted according to the aforementioned transmission system has a transmission mode discriminated in any case, according to the luminance signal and not to a color information, therefore when displayed on a display, the important portion or human face, for example, on a picture becomes low in resolution, and if so, then the picture quality seems to be considerably degraded.